BY CHRIS DALY
Having spent the past year discussing curve-related crashes and rollovers, it’s time to examine a real-life case study.
I should point out that whenever I discuss case studies, I’m usually swamped with e-mails and comments from “keyboard commandos” throughout the world. Typically, the keyboard commandos will tell me that I don’t know what I’m talking about and how dare I question the events of an actual crash.
- Fire Apparatus Rollovers, Part 11: Curves and G-Force
- Fire Apparatus Rollovers, Part 10: Old Guy Rules
- Apparatus Rollovers, Part 9: Wet Road Friction
That said, I do not use case studies to attack, ridicule, or otherwise demean anyone who may have been involved in an actual incident. Instead, I am simply trying to demonstrate the concepts and theories that we have been discussing over the past few months in a real-life environment. The goal of these discussions is to prevent a similar incident from happening again.
I will also say that most of my discussions about real-life crashes are limited in nature. This is because few fire departments are willing to share information related to a crash. When I ask for details such as police reports, scale diagrams, and witness statements, I am almost always met with silence. Therefore, I must work with what I have. If anyone would like to share the details of a crash so that we can all learn from it, please feel free to send them along.
The incident we will discuss this month occurred several years ago and involved an aircraft rescue firefighting (ARFF) vehicle at Hartsfield-Jackson Atlanta International Airport. The Atlanta (GA) Fire Rescue Department conducted an extensive investigation related to this crash and a discussion of the investigation is posted on the Oshkosh Website.1 The Atlanta Fire Rescue Department should be credited for sharing the results of its in-house crash investigation so the rest of the firefighting community can learn from it.
This crash occurred on September 23, 2008, at 1130 hours, and according to the investigative report, the weather was clear and sunny. The vehicle involved was an Oshkosh Striker 3000 ARFF unit, which was equipped with 3,000 gallons of water, 420 gallons of foam, and 1,000 pounds of extinguishing agent. At the time the crash occurred, the vehicle was driving toward a taxiway as part of an emergency response drill.
As the apparatus responded to the simulated emergency, it traversed a nonlicensed vehicle road that encircles the airport. As the apparatus traveled along this road, it drove through a security checkpoint and then entered a left-hand curve with a moderate downhill grade. As the apparatus entered the curve, the rear wheels began to lose traction, and the rear of the vehicle began to slide to the right. As the rear of the vehicle slid to the right, the right rear tires left the roadway.
When the right rear tires left the roadway, the fire apparatus operator attempted to correct the skid. When this occurred, the vehicle began to skid in the opposite direction, and the rear of the vehicle began to skid “precipitously” back to the left. As the vehicle slid toward the left, the vehicle yawed perpendicular to the centerline of the road and was now skidding on the downhill grade. At this point in the crash sequence, the vehicle rolled over onto its roof, resulting in moderate injuries to the firefighters onboard.
To begin our discussion as to why this crash occurred, let’s first examine the vehicle in question. According to advertisements, the Oshkosh Striker 3000 has been successfully tilt tested to 30.44 degrees. This tilt-table result is equal to a rollover threshold of 0.587 g. In other words, the Striker 3000 can absorb 0.587 lateral g before it begins to roll over. This is an important point, and we will come back to that.
Next, let’s examine the location of the crash. According to satellite imagery, the measured radius of the curve where this crash occurred is approximately 232 feet. Using this curve radius, we can calculate the amount of lateral g-force a vehicle would experience while rounding the curve at different speeds. The results of this analysis are provided in Table 1.
In addition to the curve radius, we need to consider the stickiness, or drag factor, of the roadway. As we have discussed in previous articles, the average drag factor for a dry road is around 0.75. However, this drag factor is lower for a truck tire. This is because a truck tire is designed to carry a heavy load over a long distance without wearing out. As a result, a truck tire will tend to be made of stiffer rubber, and it will not grip the road as well as a passenger car tire. Published studies have shown that the drag factor of a truck tire will tend to be around 85 percent that of a car tire.2 In that case, the drag factor for the truck tires on the ARFF rig would have been around 0.63. Very interesting! Remember that.
Having examined the vehicle and the roadway, let’s now examine the facts of the crash using a friction circle. As we have discussed, the available drag factor for the tires on the ARFF vehicle would have been around 0.63. As we know from our friction circle discussions, the radius of the circle is the drag factor of the road. Therefore, the radius of our friction circle will be 0.63 (Figures 1 and 2).
Figure 1: The radius of our friction circle will be the drag factor of the road. For the purpose of our discussion, we will assume that the drag factor of the airport access road was 0.75, which is the average drag factor for a dry asphalt road.
Figure 2: Studies have shown that truck tires do not grip the road as well as a typical passenger car tire. This is because truck tires are made of a stiffer rubber to prevent them from wearing out too quickly. The grip of a truck tire will typically be around 15 percent less than that of an average car tire. Therefore, we will reduce the radius of our friction circle by 15 percent, which equals 0.63.
Having drawn the radius of our friction circle, we can now plot the amount of lateral g-force the apparatus would have experienced as it rounded the curve. According to video evidence, the vehicle was traveling 47 miles per hour (mph) when it began to lose traction with the roadway. When we look at Table 1, we see that at 47 mph, the ARFF rig would have experienced 0.63 g as it rounded the curve. This amount of lateral g-force would have put the ARFF rig just at the edge of its friction circle (Figure 3). It is no surprise that when the apparatus rounded the curve at 47 mph, the rear tires began to lose traction.
Figure 3: The ARFF apparatus rounded the curve at 47 mph. A vehicle traveling 47 mph around this curve will generate 0.63 lateral g. When we plot this g-force on the friction circle, we see that tires on the fire apparatus had used up all of the available grip and began to break traction.
We should also keep in mind that the advertised rollover threshold of this ARFF vehicle is 0.587 g. While it is difficult to tell without more information and photographs from the crash scene, it is likely that at 47 mph, the vehicle was not only starting to lose its grip on the road but also starting to roll over. This is because the amount of lateral g-force the driver generated as he rounded the curve was in excess of the vehicle’s rollover threshold.
Once the vehicle began to lose traction and slide off the road, the driver attempted to counter-steer. When the driver counter-steered, he created a new curve radius in the opposite direction. The speed of the vehicle, combined with the sharp counter-steering angle of the steering wheel, once again created a lateral g-force that was greater than the rollover threshold of the vehicle. As a result, the vehicle flipped over onto its roof.
The primary causative factor for this crash was the driver traveling at a speed that was too fast for conditions. The posted speed limit for this airport access road was 25 mph. Had the vehicle rounded the curve at 25 mph, it would have experienced 0.18 lateral g, which is well below the rollover threshold of the vehicle and well within the friction circle (Figure 4). However, at 47 mph, the driver created a lateral g-force that exceeded the available grip of the tires and was also in excess of the vehicle’s rollover threshold. As a result, the vehicle lost control and ended up on its roof.
Figure 4: Had the driver maintained the posted speed limit of 25 mph, the vehicle would have only experienced 0.18 lateral g. This would have been well within the friction circle and well below the vehicle’s rollover threshold.
This case study is an excellent example of the concepts we have been discussing for the past year. Fire apparatus operators must remember to slow down, especially while rounding a curve to reduce the amount of g-force acting on the vehicle. Driving too fast for conditions will create excess g-force, which will likely lead to disaster.
2. Stopper, David A. “Heavy Air Braked Vehicle Accident Reconstruction”. Texas Engineering Extension Service, Texas A&M University, College Station TX.
CHRIS DALY is a 22-year police veteran, serving as a patrol supervisor in West Chester, Pennsylvania. He has served 29 years as both a career and volunteer firefighter, holding numerous positions, including the rank of assistant chief. He is an accredited crash reconstructionist (ACTAR #1863) and a lead investigator for the Chester County (PA) Serious Crash Assistance Team. Daly is a member of the Fire Apparatus & Emergency Equipment Editorial Advisory Board. Daly has also developed an emergency vehicle driver training program called “Drive to Survive,” which has been presented to more than 22,000 firefighters and police officers at more than 500 emergency service agencies across the United States.